1. Field of the Invention
The present invention relates to fuel cell systems and control methods therefor, and more specifically, to a fuel cell system in which aqueous fuel solution is supplied directly to fuel cells, and also relates to a control method therefor.
2. Description of the Related Art
In the field of direct methanol fuel cell systems, there has been a requirement to shorten a time from operation start to a time when the system begins operating at its full capacity. As a solution, proposals have been made with a goal of shortening a heat-up time required to heat aqueous methanol solution supplied to the fuel cell to a predetermined temperature.
For example, WO2006/025321 discloses a technique that an amount of aqueous fuel solution stored in the aqueous solution tank is moved to another tank when the system is started, in order to shorten the time required to attain a predetermined temperature.
Also, JP-A2006-004868 discloses a technique of providing a first container for storing low-concentration fuel which is mainly for power generation and a second container for storing high-concentration fuel which is mainly for power generation and increasing the temperature of the power generation cell so that one of the containers is selected thereby switching the types of fuel to be supplied to the power generation cell based on the temperature of the power generation cell.
According to WO2006/025321, it is possible to shorten the heat-up time by moving aqueous fuel solution from the aqueous solution tank to another tank thereby decreasing the amount of aqueous fuel solution which is in circulation during the heat-up operation. However, in order to begin normal operation, the aqueous fuel solution which was moved to the other tank must be returned to the aqueous solution tank, and this complicates an operation process.
Also, according to JP-A 2006-004868, it is possible to raise the temperature of the power generation cell in a short time. However, switching to the other container in order to shift to normal operation causes a sudden change in the temperature and concentration of the aqueous fuel solution which is being supplied to the power generation cell, making it impossible to perform a smooth shift to the normal operation.